Conventionally, in order to enhance the resolution of a video signal from a color image pickup device such as a CCD, filter-transmitted signals from CCD elements each having a color filter is subjected to arithmetic operation with peripheral pixels to generate luminance signals at positions corresponding to the respective CCD elements and, thereafter, resolution conversion is carried out using these luminance data. Hereinafter, a conventional CCD and signal processing thereof will be described with reference to FIG. 2.
FIG. 2 is a block diagram illustrating a conventional color image pickup device. In the figure, 201 denotes an optical system which forms an image of an object on the surface of a CCD 202. The CCD 202 converts the object image so formed into an electric signal, and a color separation filter is disposed on each CCD element to take out color data. A signal 203 outputted from the CCD 202 is an analog quantity, and each element of the CCD 202 has a voltage value according to the intensity of light transmitted through the corresponding color separation filter. 204 denotes an A/D converter which digitizes the analog signal 203 to convert the analog signal 203 into a signal 205 having 256 levels of gradation (0˜255) for each pixel. 206 denotes a storage circuit which stores the digital signal 205 and permits random access to each value, to permit arithmetic operation with peripheral pixels in the signal processing that will be described later. 208 denotes a signal processing circuit which performs processing to obtain luminance values and color-difference signals from the digital signal 207 stored in the storage circuit 206.
The process steps up to mentioned above are generally performed by the conventional color image pickup device 210, and the signal processing circuit 208 outputs the luminance values and color-difference signals 209 to the outside. When higher resolution is desired, a resolution conversion circuit 211 performs interpolation using the luminance values and color-difference signals 209 outputted from the signal processing circuit 208 to generate luminance signals and color-difference signals of pixels to be newly generated, and outputs the luminance values and color-difference signals 212 larger in number than the luminance values and color-difference signals 209 outputted from the signal processing circuit 208.
Next, the signal processing performed by the signal processing circuit 208 will be described specifically with reference to FIG. 3. FIG. 3 is a diagram illustrating an array of color filters on the CCD 202 in the conventional color image pickup device. In FIG. 3, Mg indicates elements with magenta filters, G indicates elements with green filters, Cy indicates elements with cyan filters, and Ye indicates elements with yellow filters. On the CCD 202, these elements are arranged in a checker pattern, and a pattern in a region enclosed with a bold frame is repeated.
The signal processing circuit 208 obtains luminance values and color-difference signals from the signals transmitted through these single-color filters. For example, when the luminance value at a pixel 301 is to be obtained, the average of the luminance values of four pixels 301(Mg), 302(G), 305(Cy), and 306(Ye), i.e., (Me+G+Cy+Ye)/4, is obtained in approximation, and this average is regarded as the luminance value at the pixel 301. Further, also when the luminance value at the pixel 302 is to be obtained, in like manner as mentioned for the luminance value of the pixel 301, the average of 2×2 pixels (302, 303, 306, 307) with the pixel 302 in the upper left corner, is obtained, and this average is regarded as the luminance value at the pixel 302.
As described above, luminance values in one-to-one correspondence to the respective pixels of the CCD can be obtained by calculating, in any case, the average of 2×2 pixels having the target pixel in the upper left corner and including each of Mg, C, Cy, and Ye.
Further, a color-difference signal is specified by Cr indicating a difference between a red component and a luminance value, and Cb indicating a difference between a blue component and the luminance value, and a Cr signal is obtained by (Ye+Mg)−(Cy+G) while a Cb signal is obtained by (Cy+Mg)−(Ye+G). Generally, a color-difference signal comprising a pair of Cr and Cb is provided for every four pixels because the human eyes are relatively insensitive to the resolution relating to colors.
As described above, the luminance values and color-difference signals 209 outputted from the conventional color image pickup device 210 are composed of the luminance values having resolutions in one-to-one correspondence to the respective pixels of the CCD 202, and the color-difference signals each comprising a pair of Cr and Cb and having ¼ resolutions with respect to the pixels of the CCD 202.
Next, a description will be given of the processing contents by the resolution conversion circuit 211 that receives the luminance values and color-difference signals 209 and performs resolution conversion, with reference to FIG. 5. Although many methods for resolution conversion have been proposed, linear interpolation using peripheral pixels is most commonly used. FIG. 5 is a diagram for explaining linear interpolation performed by the resolution conversion circuit 211 of the conventional color image pickup device. In the figure, G denotes a pixel to be newly formed, A denotes a pixel position on the CCD 101 that is closest to the new pixel G, B to E denote pixel positions on the CCD 101 that are adjacent to the pixel A, and F denotes a pixel position on the CCD 101 that is adjacent to the pixels C and D. Further, i denotes a distance to the pixel G in the horizontal direction when the pixel A is the target pixel, and j denotes a distance to the pixel G in the vertical direction when the pixel A is the target pixel.
Since linear insertion is carried out in linear interpolation, when the distance between the respective pixels is 1 and 0≦i<1 and 0≦j<1, the luminance value and the color-difference signal of the new pixel G are obtained as follows:G=(1−i)((1−j)A+jC)+i((1−j)D+jF)
Meanwhile, Japanese Published Patent Application No. Hei. 7-93531 discloses another method for resolution conversion wherein, simultaneously with linear interpolation, edges are specially processed so that the edges are not blurred, and the processed edges are superposed on the result of linear interpolation.
However, the above-mentioned conventional techniques have the following drawbacks. First of all, in the conventional linear interpolation, since averaging with the peripheral pixels is carried out, image is undesirably smoothed, resulting in blurred image that lacks sharpness even at edges.
On the other hand, although the process disclosed in Japanese Published Patent Application No. Hei. 7-93531 provides relatively favorable processing results, arithmetic operation for edge formation is needed in addition to that for linear interpolation, resulting in longer processing hours, or increased cost when the process is implemented by hardware.
Moreover, in either of these conventional methods, when interpolating, for example, a point placed between the pixels 301, 302, 305, and 306 shown in FIG. 3, the luminance value of the pixel 306 is used as an element for the interpolation. However, since the luminance value of the pixel 306 was generated using the luminance values of the pixel 311 and others, the pixel to be generated by the interpolation is influenced by the very distant pixels. This means that the value of the new pixel obtained by the interpolation consequently becomes a value obtained by smoothing the image over a wide range, and the image is blurred.
The present invention is made to solve the above-mentioned problems and has for its object to provide a high-resolution color image pickup device that can provide favorable processing result having no blur, with a very small amount of arithmetic.